1. Field of the Invention
The present invention relates to audio reproducing apparatus and, particularly to an audio reproducing apparatus having an audio signal reproducing unit that the listener wears on the head.
2. Background of the Invention
Audio signal reproducing methods using a headphone are known to reproduce an audio signal with the headphone worn by the listener on the head such that the headphone covers listener's ears. Thus, the listener can listen to reproduced sounds of the audio signal from the ears. When the audio signal is reproduced by the headphone as described above, even if a signal from a signal source is of a stereo signal, there occurs a so-called lateralization in which the listener feels reproduced sound image around or within the head.
A binaural pickup sound wave reproduction system is known as one of the audio signal reproducing systems using the headphone. The binaural pickup sound wave reproduction system will be described below.
According to the binaural pickup sound wave reproduction system, microphones called dummy head microphones are fitted into ears of a dummy head which assumes a listener's head. The dummy head microphones pick up an audio signal from a signal source. When the audio signal thus picked up by the dummy head microphones is reproduced by the headphone in actual practice, the listener can obtain presence as if the listener listened to sounds generated from the sound source directly. According to the binaural pickup sound wave reproduction system, it is possible to improve sense of direction, sense of localization and presence of picked-up and reproduced sound image. However, in order to reproduce the audio signal according to the binaural reproduction system, a special signal source should be provided as a signal source to provide a special sound source signal different from that used in the case that the audio signal picked up by the dummy head microphone is reproduced by a speaker apparatus.
Therefore, it is proposed to apply the binaural pickup sound wave reproduction system to a reproducing system to achieve a reproduction effect in which sound image is localized at the position outside the listener's head, e.g., speaker position similarly when a stereo signal is reproduced through the headphone by the speaker apparatus. However, in case the stereo signal is reproduced by using the speaker apparatus, if the listener changes the direction of the head or face, then the absolute direction and position of sound image are not changed but the relative direction and position of sound image that the listener feels are changed. In case the audio signal is reproduced by means of the headphone according to the binaural pickup sound wave reproduction system, if the listener changes the direction of the head or face, the relative direction and position of the sound image that the listener feels are not changed. As a consequence, if the listener changes the direction of the head or face while the audio signal is reproduced in the binaural reproduction system, then sound field is formed within the listener's head. In particular, it is thus difficult to localize sound image in front of the listener. Furthermore, in that case, sound image tends to be localized above the listener's head.
According to a headphone reproduction method described in Japanese published patent publication No. 42-227, the following binaural reproduction method using a headphone is proposed. Specifically, sense of direction and sense of localization of sound image are determined based on some suitable elements, such as a difference of volume of sounds picked up by listener's left and right ears, a time difference, a phase difference or the like. Therefore, according to the above-mentioned reproduction system described in Japanese published patent publication No. 42-227, audio signal lines of left and right channels are provided with level controllers and variable delay circuits. The level controllers and the variable delay circuits of the audio signals of the respective channels are operated under the control of a detected signal obtained when the direction of the listener's head is detected.
However, according to the headphone reproduction method described in Japanese published patent publication No. 42-227, a motor is directly driven by the detected signal obtained when the direction of the listener's head is detected. When the motor is driven by the detected signal, variable resistors and variable capacitors in the level controllers and the variable delay circuits are mechanically controlled by an analog signal. As a result, volume differences and time differences of the audio signals of the respective channels supplied to the headphone after the listener changed the direction of the head should not be changed without a delay of time so that the mechanical motor cannot cope with the motion of the listener's head.
Furthermore, according to the headphone reproduction method described in Japanese published patent publication No. 42-227, when volume differences and time differences are changed, it is necessary to determine changing characteristics of volume differences and time differences based on relative positional relationship between the sound source and the listener, the shape of listener's head, the shape of auricles or the like. Specifically, when volume difference and time difference are changed in accordance with a certain one changing characteristic, positional relationship between the sound source and the listener is fixed and therefore the sense of distance and a distance between the sound sources cannot be changed. Furthermore, since listeners' heads and listeners' auricles are different, degree of effect that the sense of direction, the sense of localization and presence of the picked up and reproduced sound image that the listener felt can be improved is fluctuated.
Japanese published patent publication No. 54-19242 describes a stereo reproduction system in which relationship of changing amounts between the direction of the listener's head and volume differences and time differences of audio signals of respective channels supplied to the headphone can be successively calculated to process the audio signals.
However, in the stereo reproduction system described in Japanese published patent publication No. 54-19242, there should be provided a memory of a vehemently large storage capacity to continuously calculate and memorize relationship of the changing amounts of volume difference and time difference of the audio signals. It is therefore extremely difficult to realize the above-mentioned stereo reproduction system.
In an audio reproducing apparatus described in Japanese laid-open patent publication No. 01-112900, there is described an apparatus which calculates relationship between the changing amounts of volume difference and time difference of these audio signals not successively but discretely to thereby process the audio signals.
The audio reproducing apparatus described in Japanese laid-open patent publication No. 01-112900, however, describes only a theoretical concept with which the audio reproducing apparatus can be applied to both the analog signal processing and the digital signal processing. Accordingly, the above-mentioned audio reproducing apparatus lacks in concreteness required when the audio reproducing apparatus is applied to the commercially-available products by using the analog or digital signal processing.
Any one of the above-mentioned headphone reproduction method, the stereo reproduction system and the audio reproducing apparatus cannot process these audio signals at high speed in response to the motion of the listener's head without the digital signal processing. However, any one of the above-mentioned headphone reproduction method, the stereo reproduction system and the audio reproducing apparatus does not describe the digital signal processing means and the digital signal processing method at all. Therefore, it is difficult to realize the above-mentioned headphone reproduction method, the stereo reproduction system and the audio reproducing apparatus.
Furthermore, it is necessary to provide the memory of large storage capacity to memorize relationship, such as the changing amounts between the volume differences and time differences of these audio signal. Although these relationships cannot be memorized in the memory without the digital signal processing, the digital signal processing means and the digital signal processing method are not described at all. Therefore, it is extremely difficult to realize the headphone reproduction method, the stereo reproduction system and the audio reproducing apparatus.